Leapseconds

A leap second is a one-second adjustment added to Coordinated Universal Time (UTC) to keep civil time within close alignment with mean solar time, UT1, which is based on Earth's rotation. The adjustment is decided and announced by the International Earth Rotation and Reference Systems Service (IERS) and is inserted at the end of June 30 or December 31 as needed.

Leap seconds were introduced in 1972 and have been added irregularly since then. A total of 27 leap seconds have been added to UTC since 1972, with the most recent one on December 31, 2016. All leap seconds to date have been positive (increasing the length of a day by one second); negative leap seconds have never been used. The purpose is to keep UTC within about 0.9 seconds of UT1, so that civil time remains aligned with Earth's rotation.

Impact and context: leap seconds affect computers, networks, and timekeeping systems, which must account for the occasional one-second insertion. Some systems implement leap-second smearing or other strategies to avoid a sudden jump, while others rely on UTC time with explicit leap-second handling. There has been ongoing debate about reforming or abolishing leap seconds in favor of a continuous time scale, or adopting alternative approaches such as smoothed adjustments; no final international decision has been implemented as of now.